Computed tomography (CT) has been widely used in medical diagnosis. CT is a technology that makes use of computer-processed combinations of X-ray images taken from different angles of an object to produce cross-sectional images. Angle information (e.g., rotation angle) of CT may be important in CT image reconstruction. During a CT scan, a CT scanner may acquire a plurality of projection samples to keep track of rotation angles of a radioactive scanning source. The CT scanner may record a plurality of projection acquisition times for the plurality of projection samples. However, because of a system error of the CT scanner (e.g., an error of hardware of the CT scanner, an error of data transmission of the CT scanner, etc.), there may be an error in the recorded projection acquisition times, which may cause an error in the determination of rotation angles. The error in the rotation angles may cause artifacts (e.g., staircase artifacts) in a reconstructed CT image. Therefore, it is desirable to provide systems and methods for determining rotation angles to reduce/eliminate the error in the recorded projection acquisition times.